Ambulatory infusion apparatuses are known in the art for a variety of applications. In particular, ambulatory infusion apparatuses adapted for insulin administration form a basis for a state-of-the-art therapy of diabetes mellitus by CSII (Continuous Subcutaneous Insulin Infusion). Those devices are typically computer controlled micro dosing pumps which are adapted to be worn continuously and concealed from view. Such insulin dosing pumps are manufactured, e.g., by Disetronic Medical Systems, AG, Switzerland, and are commercially available as Accu-Chek® Spirit. Those devices are of the syringe-driver type and comprise a typically cylindrical drug cartridge out of which insulin is forced into an infusion line by displacing a cartridge plunger in a controlled manner.
For a variety of technical as well as convenience and application-related reasons, recent systems are based on a different fluidic architecture, for example according to the disclosure of the European patent application EP 1970677A1. For such and some further designs, a drug cartridge as known from state-of-the-art systems is less favorable and may therefore be replaced by a flexible container. Such flexible containers are typically made of two, for example, circular or rectangular sheets of an elastic foil, in which the sheets are bonded in a circumferential area. Further design aspects and materials for such flexible containers are disclosed, among others, in the co-pending European patent application 08167548.0. For illustrative purposes, FIG. 1 shows a flexible container of this type in its filled state. The flexible container 20 is made of two elastic foil sheets 21, 21a which form a container body and comprises a fluidic connector 30. Variations of the design without changing the overall operation principle are obvious for a person skilled in the art. For example, instead of two separate sheets, one folded sheet may be used for the container body. In a further variant, the container body may be partly rigid and be made, for example tank-like and have a cover or top made from a flexible foil sheet. The embodiments of the present invention may be used in connection with all of those designs.
It is to be appreciated that a flexible container made of two elastic foil sheets may be substantially flat in its empty state and assume a ‘cushion-like’ shape upon being filled. The main direction of expansion is substantially normal to a plane which is defined by the empty container. The expansion is associated with a strain of the foil sheets which increases with the filling volume.
When the drug reservoir of an ambulatory infusion apparatus is filled by a patient, costs and convenience are major issues. Particular problems result from the fact that ambulatory infusion devices are typically operated by the patient himself or herself, i.e., a person without special skills in the fields of medicine or medical technology. In many cases, diabetes is accompanied by further diseases or handicaps such as severe ametropia and/or tactile distortions. Therefore, simple handling is of utmost importance. The drug containers are typically single-use items and are designed to hold the drug amount of one week or less. The process of filling and replacing the drug container accordingly has to be carried out quite often by the patient.
Furthermore, it has to be understood that drug containers are often not filled to the maximum amount. The patient typically fills the drug container to a volume in dependence of his or her individual insulin demand as well as the maximum storing time in the container which is limited. Especially when stored in a container made of plastics or plastic foils, drugs such as insulin formulations are known to be stable for a limited time only before they substantially degrade. In addition to time, the environmental temperature is known to have impact on the degradation. Therefore, diabetics needing comparatively little insulin per time period, such as children, as well as patients living in zones of high temperature, often do not fill the drug container to the maximum specified filling volume.
In contrast to a syringe-like drug container, filling beyond the specified maximum filling volume is well possible for a flexible container. Overfilling results in an increased leakage danger and may prevent the flexible container from smoothly fitting into a corresponding container compartment of the infusion apparatus.
During application of a drug container in an ambulatory infusion apparatus, monitoring of the drug amount remaining in the container is required in order to alert the user in due time before the reservoir is empty. For a cylindrical cartridge having a displaceable plug, the plug position within the cartridge is proportional with the filling volume. By determining the plug position or the plug displacement with respect to an initial reference position, the remaining drug volume may accordingly be obtained. In contrast to this, such a simple geometric measure is not available for flexible containers for alarming the user about the container becoming empty. It is therefore desirable to fill those containers to a well defined initial filling volume, such that the remaining drug volume can be computed from the initial drug volume and the drug volume already administered.